Measuring electro-adhesion pressure before and after contact

Electro-adhesion (EA) is a low-power, tunable, fast and reversible electrically-controlled adhesion method, effective on both conducting and insulating objects. Typically, only the electro-adhesive detachment force, i.e., the force required to separate an object from the EA patch, is measured. Here, we report a method that enables comparing the pre-contact EA attachment forces with post-contact EA detachment forces. We observe that pre-contact pressures are 1 to 100 times lower than post-contact detachment pressures, indicating the large role played by surface forces, charge injection, and polarization inertia. We characterize the time-dependence of pre- and post-contact EA forces as a function of the applied voltage waveform, observing that using an AC drive allowing for much faster release than DC operation. We measure both EA forces on conductive and insulating objects, using over 100 different EA patches covering a wide range of electrode dimensions. At 400 V, the EA release pressures for conductive objects range from 1 to 100 kPa, and are 1 to 10 times higher than pre-contact adhesion force. For dielectric objects, release pressures are 1 to 100 higher than pre-contact adhesion pressures. The methodology presented in this paper can enable standardized EA characterization while varying numerous parameters.

www.nature.com/scientificreports/ etc. Regardless, methods to measure EA forces share several common steps. As described in Fig. 2a, a typical process for measuring normal forces consists in (a) an approach phase to bring the EA patch into contact with the object, (b) a contact phase, and finally (c) the detachment phase, during which the release force is measured. During the contact phase, a wait time of 10 s, 60 s and 90 s is often used 5,7,9,14 . This holding time has a significant impact on release EA pressure, greatly increasing the holding force, but slowing the release when the voltage is turned off. The holding times correspond to the most obvious use cases for EA patches in grippers 9,17,18,21 and as adhesive surfaces for robotics 6,15,16,24 . A force relaxation of order 20 mN is observed during the holding time, which we attribute primarily to the viscoelasticity of the insulating layer between the interdigitated electrodes and the object 25 . Figure 2b, c, and d depict various photographs of the fabricated wafer, the test bench, and the object on the force sensor.
The "electro-adhesive pressure" reported in all papers to date is the release pressure. We extend here the measurement process to record force data also before contact between the EA patch and the object, during the holding time phase, in addition to the commonly reported detachment phase. We can thus measure both (i) the pre-contact adhesion force, which is the maximum force while the object approaches the EA patch, and (ii) the release force, the maximum force when detaching the object from the EA patch. We observe EA release pressures between 1 to 10 times higher than adhesion pressures for conductive objects, and 1 to 100 times higher for dielectric at 400 V. Surface contact forces, charge injection, and slow polarization can be much larger than the "pure" EA force, which is the only force we measure in the pre-contact case.

Methods
We carry out two sets of measurements. The first study, "Effect of Waveform and Holding time", investigates the impact on both the adhesion and the release pressure of AC and DC waveforms with holding times of 0, 10, 30 and 90 s, for one given dimension of electrodes. The second study, "Adhesion versus Release Pressure", reports the adhesion pressure and the release pressure, for electrodes driven by a 400 V AC bipolar square wave, for a broad range of electrode dimensions, from 5 µm to 500 µm, with no additional holding time during contact. These measurements were made using the same experimental setup.
Test bench. The test bench measures the force between fixed EA patch fabricated on a glass wafer and an object (insulating or conductive) moving on a motorized stage. We move the object down until contact (attachment), then pull it up (release). Each measurement cycle thus provides: (a) during the approach: the electroadhesive force before contact, i.e., avoiding any dependence on tackiness of the insulator and other surface forces, and (b) on the way up: the EA force required to pull the object off of the electrodes.
We measured the EA pressure on more than 180 microfabricated EA patches. To date, the lowest gap and width for EA electrodes was 80 µm, reported by Wang et al. 26 in 2012. In order to characterize electrodes with smaller dimensions, in view of lowering the operating voltages, and to compare with literature, our interdigitated electrodes have widths and gaps ranging from 5 µm to 500 µm. Our smallest devices are up to an order of magnitude smaller than typical EA structures (see SI section S1).
The EA electrodes are 80 nm thick gold, patterned on 100 mm diameter Borofloat® wafer. Each wafer holds 30 sets of interdigitated electrodes, as shown in Fig. 2c. Microfabrication consisted of metal deposition, photoresist (a) EA test bench consisting of a force sensor, a target object (conductive or dielectric), an EA patch with interdigitated electrodes, and a multi-axis stage (b) Electroadhesion for conductive and dielectric objects. The EA force (red arrow) is normal to the EA patch in both cases, but the electric field distributions are different, with fringing fields playing an essential role in the case of a dielectric object. www.nature.com/scientificreports/ coating, direct laser writing, Argon ion etching, and finally photoresist (PR) stripping (details are provided in SI section S2). After electrode fabrication, we blade casted the insulator P(VDF-TrFE-CTFE) on the wafer, with final insulator thicknesses t of 6 µm and 20 µm. We do not dice the wafers. We chose P(VDF-TrFE-CTFE) (poly vinylidene fluoride, trifluoroethylene, 1,1-chlorotrifluoroethylene) as our insulator due to its high relative permittivity of approximately 40, and because of the high EA forces it enabled in electrostatic clutches 27,28 . This polymer has a breakdown field above 120 V.µm −1 , which allows us to operate reliably at 400 V even for a thickness of 6 µm. The objects to be adhere to (both conductive and insulating) used in this study were flat, with sub-µm roughness (see SI section S3). The conductive object was a gold-coated Silicon chip of dimensions 5 mm × 5 mm. The dielectric object was a 5 mm × 5 mm glass chip, diced from a Borofloat® wafer.
The test bench consists of a 4.4 N force sensor (Low Profile Load Cell, LRF400 from FUTEK) attached to a motorized platform enabling vertical translation of the object relative to the wafer. The wafer is clamped on a tip/ tilt platform to ensure parallelism with the object, with 0.03° angular resolution. This tip/tilt precision is required because an angular misalignment between the object and the patch leads to significantly lower measured pressure (see SI section S4). An XY stage allows translating the wafer under the force sensors while maintaining angular alignment. The automated motorized z-stage motion ensured repeatability of the measurements.
Electrical connections to the EA patches on the wafer were made using probe needles (see Fig. 2b and c). Current and voltage are measured continuously using an oscilloscope equipped with a high voltage probe. The voltage is supplied by a high voltage amplifier (Trek 609E-6) driven by a signal generator. When comparing AC and DC waveforms, the maximum voltage was 2 kV. The AC signal is a symmetrical bipolar square wave, to minimize dielectric charging while keeping a constant Maxwell pressure in time. For the "Adhesion versus Release Pressure" study, the maximum voltage we used was 400 V (due to breakdown in the smallest gaps at higher voltages).
Measurement process for pre-and post-contact EA pressure. Our method is designed to measure both adhesion and release force. The EA pressure was computed by dividing the recorded force value by the surface area of the object, which was taken as 25 mm 2 for both conductive and dielectric objects. As illustrated in www.nature.com/scientificreports/ an optional stationary holding phase: the object remains in contact with the patch for a set time (from 0 to 90 s) with the voltage on. (iii) the detachment phase: with the still voltage on, the object is raised up to its original position, with sudden detachment occurring during this phase. Force, voltage and current data are recorded continuously during all phases. As plotted in Fig. 3b, we see two peaks in EA pressure during the measurement. As the object nears the EA patch during the approach, the electro-adhesive force increases: the object is pulled down towards the EA patch. The positive peak in force is the maximum "contactless" EA force which correspond to the "pure" EA adhesion force. Once the object touches the EA patch, the object (and force sensor) is compressed, leading to a negative force reading. We stop the stage motion when the compressive force reaches − 200 mN, in order to avoid damaging the patch or object by pushing them too hard together. The exact value of this maximum compressive force (eg 150 mN or 300 mN) has negligible influence on the peak EA forces. We hold the contact position during the www.nature.com/scientificreports/ holding phase for 0 to 90 s, during which charge injection and slow polarization can occur. The stage is finally raised during the detachment phase. The force decreases in absolute value, passes zero (i.e., becomes attractive, showing adhesion), increases and then suddenly drops to zero once the object detaches from the EA patch. All displacements were at a speed of 3 µm/s. This measurement method allows quantitatively comparing adhesion and release pressures, and thus distinguishing contact forces from EA forces. We varied several EA parameters including object type, insulating layer thickness, holding time or signal waveform.
EA pressure data. We define the adhesion pressure as the maximum pressure reached during the downward phase (i.e., pre-contact) and the release pressure as the maximum pressure reached during the upward phase (see Fig. 3b). Figure 3b plots in blue the measured force versus time. In Fig. 3c, we schematically split the measured pressure into a "pure" EA pressure, plotted in red, and reaction force and dry adhesion pressure in green and in orange.
The red plot schematically represents the EA force, which increases during the downward phase as the electric field at the object surface or in the object increases as the object nears the EA patch. Then once in contact, at a constant position, the EA force likely increases due to charge injection, polarization, Johnsen-Rahbek effect and other contact interactions. For simplicity, we assumed a linear increase over time for the figure. Finally, the EA force drops at detachment and becomes null after the object has returned to its initial distant position.
The green plot schematically represents the mechanical reaction force of EA on the object attached to the force sensor. Starting from zero well before contact, this reaction force increases in amplitude (becoming more negative) as the object pushes on the patch. After contact, the slope is proportional to the imposed displacement of the sensor, due to the stiffness of the system (dominated by the stiffness of the force sensor). In the holding stage, the reaction force increases to compensate for the EA force that is increasing due to polarization and charge injection. During the upward phase, the reaction force decreases, then switches sign (becomes attractive). We schematically plot this in orange and label it the dry adhesion force. The reaction force drops to 0 after detachment between the object and the patch.
The adhesion force corresponds to a pressure purely due to electrostatic attraction, i.e., to material polarization in the case of a dielectric object, and to charge mobility in the case of a conductive object. The release pressure includes surfaces forces and electrostatic forces from charge injection.

Experimental conditions for "Effect of Waveform and Holding time".
This study compared the EA pressure for AC and DC waveforms, for both adhesion and release. We measured the electro-adhesive pressure using the same method as described in section "EA pressure data", for voltages from 0 V up to 2 kV.
For the DC case, we added 0, 10, 30 and 90 s holding times T D (i.e., additional contact time). These experiments were performed on a sample with an electrode width of 100 µm, an electrode gap of 100 µm and an insulator thickness t of 20 µm.
Unlike earlier studies of DC residual force characterizations experiments, we report both pre-and postcontact (i.e., approach and detachment) forces, as well as the dependence of the residual forces on the contact duration 29,30 . Experimental conditions for "Adhesion versus Release Pressure" measurement at 400 V. The goal of this study is to characterize the EA pressure for two thicknesses of insulating layer for both conductive and dielectric objects, for a broad range of electrode dimensions. We used a maximum voltage of 400 V because for our smallest gaps (5 µm) and thinnest insulator layer (6 µm) the breakdown voltage of the insulator is around 500 V.
To measure the electro-adhesive pressure, each patch was tested at 0 V, then 400 V, and again at 0 V for both conductive and dielectric objects with no holding of contact. For tests on conducting objects, we used a square bipolar drive at 5 Hz. This frequency was chosen to minimize charge injection and thus additional adhesion forces. For dielectric objects, a 1 Hz bipolar waveform was used, to ensure reasonably full/complete polarization of the object. The choice of bipolar frequency is explained in the supporting information section SI5. We performed a total of 511 measurements on conductive objects and 112 for dielectric objects.

Effect of Waveform and Holding time.
In this section, we compare the EA forces on a dielectric object when driven by a unipolar DC and bipolar square AC waveforms. We also study the effect of adding an addition holding time T D while the object is in contact with the EA patch. The data shown in Figs. 4 and 5 is taken using one single geometry: an interdigitated EA patch with electrodes width of 100 µm, gap of 100 µm and insulator thickness of 20 µm. Figure 4a and b plot the measured adhesion and release pressures versus voltage for bipolar square AC (1 Hz) and DC waveform with no additional holding time between the approach phase and the detachment phase. For all conditions, the adhesion and release pressures increase with the voltage. A purely capacitive force should theoretically scale as V 2 . The good fit of the measured data to a quadratic curve confirms the V2 dependence of the EA pressure.
No significant difference in adhesion (ie pre-contact) pressures is seen between AC or DC waveforms. This is expected as charge injection cannot occur before contact. Release pressures are 1.5 higher for DC than for AC. The minimum duration of contact between the patch and the object is approximately 24 s, given the time to reach  Figure 4c and d plot adhesion and release pressures when an additional holding time T D (contact time) of 10, 30 and 90 s is used for the DC cases. The contact time has several effects. Charges can be injected into the insulator due to the high electric field, leading to a higher capacitive pressure. Close contact between the surfaces leads to high Van der Waals forces. Given the polarization inertia, the EA pressure increases over time 31 . Figure 4c illustrates that the adhesion pressures are very similar for AC waveforms and for the DC waveforms with different hold times, which is expected as no net charge injection occurs before contact. Figure 4d however shows that adding the holding time gives up to a sixfold increase in the release pressure for the DC case. At 2 kV, the release pressure increases from 5 kPa for AC drive up to almost 60 kPa for DC drive when a 90 s holding time is used. The longer the dielectric object remains in contact with the EA patch, the higher the release pressure is. www.nature.com/scientificreports/ In Fig. 5, we investigate how long it takes for the increased force stemming from a holding time decay after the voltage is set back to zero. In order to characterize the effect and time dependence of charge injection and object remanent polarization, after each 1200 V measurement, we performed our standard force versus time measurement at 0 V (as illustrated in Fig. 5a). We measured the residual pressure due to charge injection and dielectric object polarization at 0 min, 10 min and 20 min after the measurements at 1200 V, as shown in Fig. 5b and c, for residual adhesion and residual release pressures. At 0 min after the 1.2 kV measurements, residual pressures for AC waveforms are less than 0.1 kPa, while for DC waveforms, residual pressures are five times larger. With a holding time of 10 s or higher, the residual adhesion pressure increases to a maximum of 1.5 kPa and residual release pressure reaches 2.4 kPa. The residual pressure drops to less than 1 kPa after 10 min and become lower than 0.3 kPa after 20 min. Figure 6a plots the adhesion pressure (i.e., pre-contact pressure, with no surface forces) versus the release pressure (i.e., post-contact pressure, with surface forces) at 400 V for over 180 different EA patches, tested on both conductive and dielectric object, and with insulator thickness t of 6 µm and 20 µm. Each of the over 267 data points corresponds to one combination of electrode width and gap (ranging from 5 µm to 500 µm), object type and insulator thickness.

Adhesion versus release measurements at 400 V for different electrode dimensions.
Dashed lines in Fig. 6 show different ratios of release to adhesion pressure. The measured release pressure is always greater than the adhesion pressure (which is expected given the extra forces that are involved after contact is made), and the ratio is lower for conductive objects than for insulating objects. Figure 6b plots the measured release pressure versus the measured adhesion pressure for applied voltages of 100 V, 200 V, 300 V and 400 V for a conductive object and 20 µm thick insulator. Higher voltages lead to higher forces, as was more clearly shown for a single EA patch in Fig. 5.  To show overall trends, we color code widths and gap in 3 bins: smaller than 20 μm, 20 to 100 µm, and greater than 100 μm. Figure 7a plots adhesion pressure versus release pressure at 400 V, for a dielectric object, with an insulator thickness of 6 µm. The data points are colored according to the three groups of the width of the interdigitated electrodes. The highest adhesion pressures are reached on average for electrode widths between 20 and 100 μm, while the highest release pressures are obtained with width higher than 100 µm. In Fig. 7b) we have the same configuration but for an insulator thickness of 20 µm. The release pressures are similar to the 6 µm case, but the adhesion pressures are smaller. Figure 7c is similar to 7a but for a conductive object. The highest adhesion pressures are reached on average for widths higher than 20 μm. Figure 7d plots the adhesion pressure versus release pressure at 400 V, for a conductive object, with an insulator thickness of 20 µm. The maximum adhesion and release pressures are reached in average for width lower than 100 μm. Increasing the insulator thickness confined the electric field inside of the insulator and reduces the effect of the width on the EA pressure. Figure 8a plots the adhesion pressure versus release pressure at 400 V, for a dielectric object, with an insulator thickness of 6 µm. The data is grouped into 3 ranges of interelectrode gaps. Maximum adhesion and release pressures are reached on average for gaps lower than 100 μm. In Fig. 8b) we have the same configuration but for an insulator thickness of 20 µm. Both maximum adhesion and release pressures are reached in average for gap between 20 and 100 μm. Figure 8c is similar to 8.a but for a conductive object. The highest adhesion pressures are reached on average for gaps lower than 20 μm. Finally, Fig. 8d plots the adhesion pressure versus release pressure at 400 V, for a conductive object, with an insulator thickness of 20 µm. The maximum adhesion and release pressures are reached in average for width lower than 100 μm. Increasing the insulator thickness confined the electric field inside of the insulator and reduces the effect of the width on the EA pressure.
It is clearly seen in Fig. 6 that conductive objects have higher EA pressures than dielectric objects, for a given voltage and electrode geometry. Comparing Figs. 7 and 8, for a dielectric object, decreasing the insulator thickness reduces on average the release pressure and increases the adhesion pressure. In the case of a conductive object, decreasing the insulator thickness increases both adhesion and release pressure.
The scatter in our data is not due to charge injection: we know this because we take a force-displacement curve at 0 V before and after every measurement, and can thus verify the absence of residual pressure. The scatter may be in large part due to dust, and hence to an additional spacing between patch and object. To minimize this, www.nature.com/scientificreports/ we cleaned both the EA pad and the object with lint free wipes before each measurement, but the measurement was done in a standard lab environment (not in a cleanroom). For a conductive object, adhesion pressures range from 1 to 15 kPa and release pressures range from 1 to 100 kPa. No clear difference is seen between insulator thickness t of 6 µm and 20 µm. For dielectric object, adhesion pressures range from 0.01 to 2 kPa and release pressures from 0.1 and 10 kPa. Higher release/adhesion pressure ratio are seen for an insulating layer of 20 µm compared to 6 µm.
The ratio of release to adhesion pressure is bounded between 1 and 10 for the conductive objects, and between 1.5 and 50 for the dielectric ones, showing the importance of surface forces, charge injections, and possible longer-time scale polarization.

Conclusion
Our experimental method is unique in measuring both pre-contact EA "remote" forces as well as the larger and more widely reported post-contact release forces. When using EA for manipulation and locomotion, the release force is the practically useful value. The release force however depends on contact time and includes charge injection and surface forces, making it more difficult to tease out the effects of electrode geometry.
Over a wide range of samples, we measured release pressures at 400 V from 0.1 kPa to 100 kPa, similar to the pressures reported in the literature at voltage of several kV, but with larger electrode gaps than ours. The adhesion pressure values cannot be directly compared to prior work on EA because our experiment is the first to measure the electro-adhesive pressure during the adhesion phase, rather than the higher force observed in the detaching phase.
We measured the time dependence of EA pressures, and the resulting residual pressures. The longer one remains in contact with a DC voltage on, the higher the EA release pressure will be, and the higher the residual pressures due to charge injection and remanent polarization will be, lasting 10 to 20 min after a measurement at 1200 V. For applications such as grippers, the drive waveform could be dynamically switched between DC and AC to allow the maximum grasping force in DC while then obtaining minimal residual pressures of AC for quick release.
Complementing standard EA test benches that report only the release pressure, we extended in this paper the measurement methodology to also quantify the pre-contact adhesion force, which excludes contact forces or time dependent interactions with the objects. We used this test bench to study the impact on EA of different object nature (conducting vs. insulating), insulator thickness, and also of different waveforms (e.g., AC or DC). The influence on EA pressure of other parameters such as materials permittivity, roughness, humidity and others could also be studied using this method.
The devices reported in this article were fabricated on rigid substrates, as it was easier to perform photolithography on glass wafers. EA is however often exploited on flexible or even stretchable substrates. We expect all conclusions to hold on curved surfaces when the EA patch is sufficiently compliant to match the object shape.